Filter for transmission line



Feb 28 1956 A. G. CLAVIER ET AL I FILTER FOR TRANSMISSION LINE FiledMarch 27, 1950 lNvENToRs ANDRE G. cLAv/ER OAV/o L. THOMAS A ORNE Unit@gStates Patent Office 2,736,866 Patented Feb. 28, 1956 FILTER FORTRANSMISSION LINE Andr G. Clavier, Nutley, N. J., and David L. Thomas,

Lee, London, England, assignors to International Standard ElectricCorporation, New York, N. Y., a corporation of Delaware ApplicationMarch 27, 1950, Serial No. 152,132

13 Claims. (Cl. 3253-73) This invention relates to microwavetransmission systems and more particularly to a filter and means forcoupling the filter in a microwave transmission system.

In addition to the transmission of microwave energy over radio links,coaxial cables and dielectric waveguides, it has recently beendiscovered that high frequency energy covering a very wide band may betransmitted by means of a single wire properly insulated, thetransmission of energy being confined closely adjacent the surface ofthe wire in the electric and magnetic fields formed thereabout. It hasbeen recognized that when a single wire had electric energy appliedthereto that electric and magnetic fields formed about the wire but itwas believed that these fields extended outwardly without any definitelimits or boundary. lt has been found, however, that if the conductor iscoated with a given thickness of insulation the electromagnetic fieldsare conp centrated and substantially confined within a cylindricalvolume of a given radius about the conductor, the given radius beingdetermined by the size of the conductor and the quality and thickness ofthe insulating coating. By way of example, an ordinary No. 12 enameledcopper wire was found to have an electromagnetic field.

concentrated within a three to four inch radius about the Wire and thatthe high frequency energy fiowed in this field. This manner ofpropagation now referred to as the surface wave transmission presentsvery small loss and is substantially free from electrical and 'otherdisturbances where this confined cylindrical field is substantiallyunobstructed.

One of the objects of this invention is to provide a filter for passageof certain microwave frequencies.

Another object is to provide a combined filter and coupling arrangementfor coupling the filter to certain types of microwave transmissionconductors.

One of the features of the invention is to utilize a conductorcorresponding to the so-called surface wave conductor as a filter. Thisis done by disposing enlarged conductive sections such as discs orplates at spaced in-` tervals along the conductor, the spacing dependingupon'v the frequency energy to be passed. To avoid or minimizeperturbation in the propagation of microwave energy, the disc sectionsat the ends of the filter are made gradually smaller in diameter orcross-sectional dimension until the last one at each end is nearly thesame cross-sectional size as the Wire conductor. The filter conductorand disc sections may be bare of insulation but in order to confine theelectromagnetic field throughout the length ofthe filter, it ispreferable to coat with insulation the conductor and disc sectionssimilarly as the single wire surface wave conductor associatedtherewith. The disc sections may be circular, elliptical, rectangular,triangular, or any other shape that does not produce undue perturbationin the wave propagation.

Another feature of tne invention is the manner in which the filter iscoupled between a coaxial line or waveguide and a surface waveconductor. The end of the waveguide structure or the outer conductor ofa coaxial line is ared outwardly and the tapered end sections of thefilter are disposed at least partly within the said fiared portion. Thuswhen the electromagnetic field is permitted to extend radially in thefiared portion of the waveguide or coaxial outer conductor, the taperedfilter section or sections are disposed axially of this expanding zoneso that a minimum amount of perturbation occurs. 'This relationship ofthe tapered section or sections of the filter and the fiared portion ofthe associated dielectric waveguide or coaxial line bears out for bothdirections of wave propagation, that is, whether or not the microwavepropagation is being launched from the fiared portion or being receivedthereby.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood, by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

Fig. l is a view in side elevation of a filter according to theprinciples of this invention;

Fig-2 is a longitudinal sectional view of the same type of filter shownwith tapered end sections coupled to a surface wave conductor;

Fig. 3 is a longitudinal sectional view of the filter coupled to adielectric waveguide; and

Fig. 4 is a longitudinal sectional view showing a modified filterstructure coupled to a coaxial line.

Referring to Fig. 1, the filter basically comprises a conductor 1 whichmay correspond in size to the surface wave conductor with which thefilter is associated, although it may be of another size depending uponthe overall filter construction and how it is coupled in thetransmission system. The conductor 1 is provided with enlarged sectionsin the form of discs 2 spaced at intervals therealong. A basic filtersection comprises a pair of such discs. The discs may be circular,elliptical, rectangular, triangular or any other shape that will notproduce undue perturbation in the wave propagation. The conductor linemay also be of shapes other than the usual round form and where desiredthe conductor may be of a shape similar to the shapes of the discscarried thereby. The spacing intervals d have a direct bearing upon thefrequency band that may be passed by the filter. The upper cut-off wavelength of the filter may be represented by the formula )\c|r=g while thelower cut-off may be represented by the formula cL=d A narrow band passfilter may be constructed by using two such filters in series, one ofthe filters being proportioned as a high pass filter and the other as alow pass filter.

The above equations are effective providing the manner of propagation isnon-radiating. The thickness W being too great compared with the spacingd would result in undesired energy radiations. The thickness W shouldtherefore be kept small. The ratio of the cross-sectional dimension ordiameter D to the diameter of the conductor 1 will affect the sharpnessof cutoff and attenuation of the filter. The difference between theradius of the discs and the radius of the conductor should not begreater than one quarter of a wavelength of the upper cut-off frequency.

The filter shown in Fig. 1 is bare of insulation but may be coated withinsulation, such as enamel, glass, poly styrene, polyethylene, etc.,where it is desired to have the y electromagnetic field along the filterconfined closely 'grasses adjacent the filter sections. in Fig. 2 thefilter therein shown is provided with such an insulating coating asindicated at 3. The filter structure of Fig. 2 is provided with endportions which minimize the perturbation of the microwave propagation ineither direction with respect to the filter. The end sections are shownto comprise a plurality of discs 4, the diameters of which areprogressively diminished from adjacent one of the discs of maximumdiameter to the endmost disc 5 of the filter, the end disc 5 beingslightly larger than the diameter of the conductor 1. The conductor 1beyond the endmost section 5 may be coupled to or made integral with asurface wave line conductor 6 or may itself comprise a part of the lineconductor 6. The conductor 1 and the disc line sections of the filterare all shown to be coated with a layer of insulation 3 of a thicknesscorresponding to the insulation on the line conductor 6. It will beclear, however, that the coating of insulation on the filter conductorand sections may be either thicker or thinner than the coating on theconductor depending on the permissible diameter of electromagnetic fieldabout the filter as well as the filtering action thereof.

In Fig. 3, a filter with terminatingl tapered sections as shown in Fig.2 is shown coupled to a hollow conductor 7 which may comprise a circularwall structure of a waveguide or the outer conductor of a coaxial line.The end portion 8 of the hollow conductor may be plane but preferably isfiared in the form of a horn and certain of the end sections of the lterare disposed within this flared portion. The end lter section 5 isdisposed closely adjacent the inner end of the iiared portion 8 so thatthe taper of the end sections associated therewith correspondsproportionately to the angular degree of the ared portion 8 which isconcentrically disposed thereabout. Where the hollow conductor 7 isprovided with a center conductor 9, as in the case of a coaxial line,the insulation 3 is also tapered from the periphery of the disc section5 to the surface of the conductor 9 as indicated by the tapered body 10.The disc sections are preferably the same shape as the cross-section ofthe waveguide or coax.

In Fig. 4, a similar microwave transmission line of the coaxial type isshown as comprising an outer conductor 11 and an inner conductor 12. Theouter conductor is also flared as shown by the end portion 13. Thehollow conductor 11 contains a solid dielectric 14 which extends at thesame diameter outwardly through the flared portion 13. The filter inthis embodiment is provided with a tapered end section 15 which has amaximum diameter D corresponding to the diameter of the other sections16 of the filter. The end section 15 is tapered from its end of maximumdiameter to the surface of the conductor 12 substantially in radialalignment with the inner end 17 of the ared portion 13. The section 15is proportioned to extend for a length equal to or slightly larger thanthe length of the fiared portion 13 of the outer conductor.

In both forms shown in Figs. 3 and 4, the tapered end portion of thefilter is disposed within the flared horn end of the outer conductor.This relationship provides for smooth transfer of energy from manner ofpropagation to another with a minimum of perturbation. In Fig. 4 thedielectric body contained in the conductor 11 is gradually diminished inthickness along the section 15 until it reaches the uniform thickness ofinsulation on the filter. This gradual change in the dielectric alsoaids in minimizing the occurrence of perturbation of the wavepropagation.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatmany variations are contemplated, for example, the discs may be ofvarious shapes so long as the longitudinal cross-sections thereof aresymmetrical. Also, the discs may be made separately and secured to theconductor or they may be made integral therewith. This description istherefore to be regarded as being made only by way of example and not asa limitation to the scope of our invention as set forth in the objectsthereof and in the accompanying claims.

We claim:

l. A high frequency band pass filter for use in conjunction with aninsulated line conductor adapted singly to propagate high frequencyenergy through van electromagnetic field confined closely adjacent thesurface of such conductor, comprising a single conductor and a pluralityof groups of axially separated enlarged conductor sections extendingoutwardly about said conductor, each group consisting of a plurality ofserially connected conduct-or sections, adjacent conductor sections ofone group being spaced one wavelength apart, and adjacent conductorsections of a second lgroup being spaced two wavelengths apart, the twogroups being serially connected and the wavelength 'spacingsrepresenting the upper and lower cut-off wavelengths of said band passfilter.

2. A high frequency filter according to claim l, wherein said enlargedsections `are coated with insulating material.

3. A high frequency filter according to claim l, wherein the conductorand the enlarged sections thereof are coated with insulating material.

4. A high frequency filter according to claim l, wherein the endsections of the filter are graduated in size with the endmost sectionslightly larger than the diameter of the conductor.

5. A high frequency filter according to claim 4, wherein the conductorand the enlarged sections are coated with insulating material with theinsulation Iadjacent the end section being tapered from the periphery ofsaid end section to closely adjacent the surface of said conductor.

6. A high frequency filter according to claim l, wherein the diametersof the conductor sections are approximately equal and the end section istapered from la diameter Isubstantially equal to the diameter of theother sections down to the diameter of said conductor.

7. A high frequency filter according to claim 6, wherein the sections ofsaid filter are coated with insulation.

8. A high frequency filter according to claim l, further including ahollow waveguide conductor coupled to one end of the filter, and the endportion of said filter being disposed at least partly within the endportion of said end section being disposed substantially within the aredportion of said hollow conductor.

l1. A high frequency filter according to claim 9, wherein the endlsection is tapered from a diameter corresponding to the diameter of theother sections to the diameter of the line conductor.

12. Ahigh frequency filter according to claim 9, Wherein the endsections are of progressively smaller diameter in the direction of theendmost section.

13. A high frequency filter according to claim 9, wherein the hollowconductor -contains a body of dielectric material extending outwardly ofsaid flared portion with the end section of said filter imbeddedtherein.

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